The present invention relates to semiconductor integrated circuit devices including field effect transistors (FETs) using nitride semiconductor applicable to radio frequency (RF) transistors and methods for fabricating the devices.
Compound semiconductor of a Group-III nitride typified by gallium nitride (GaN) is wide-gap semiconductor having a large band gap and has characteristics of a dielectric strength higher than that of, for example, compound semiconductor such as gallium arsenide (GaAs) or semiconductor such as silicon (Si) and a high saturation drift speed of electrons. Accordingly, compound semiconductor has attracted attention for use in RF high-power transistors and has been intensively studied and developed.
As a result of studies in which the gate length is reduced to 0.18 μm and a recess structure is formed around a gate electrode so as to reduce parasitic resistance between a source electrode and a drain electrode, the maximum oscillation frequency fmax is increased to 140 GHz (see, for example, T. Murata et al., IEEE Trans. Electron Devices, 52 (2005) 1042).
It is recently reported that the gate length is reduced to 60 nm so as to obtain a maximum oscillation frequency fmax of 173 GHz in an FET (see, for example, M. Higashiwaki et al., Jpn. J.Appl. Phys., 44 (2005) L475). The use of such excellent RF characteristics enables application as a transistor and an integrated circuit for a submillimeter wave band in which the frequency is 20 GHz or more. In this frequency band, communication application using ultra wide band (UWB) radio communication is implementable and development into, for example, short-distance radar systems and other use is expected.
In this manner, in the case of applying nitride semiconductor hopefully expected for RF transistors to a frequency band of a submillimeter wave band or more, circuit integration including a passive element part is indispensable. In such an RF band, a configuration in which not a passive element such as an inductor or a capacitor but a so-called microstrip line formed by a metal film serving as a ground line on the entire back surface of, for example, a substrate and an metal interconnection formed on the surface of the substrate or a coplanar line formed by providing a ground line at each side of metal interconnection on a substrate, for example, is integrated as a passive element is generally adopted.
In a microstrip line, a ground line is formed on the back surface of a substrate so that the chip area is reduced. However, a process technique of connecting metal interconnection on the surface of the substrate and metal interconnection on the back surface thereof through a via hole penetrating the substrate needs to be established for a microstrip line. In the case of using a sapphire substrate widely used for crystal growth of nitride semiconductor, it generally difficult to process the sapphire substrate by dry etching, so that a through hole, i.e., a via hole, cannot be formed. Accordingly, a passive element is formed using a coplanar line, and an integrated circuit for a submillimeter wave band is implemented.
It is confirmed that a semiconductor integrated circuit device having the recess structure described above and formed by a two-stage amplifier in which an FET having a maximum oscillation frequency of 140 GHz and a coplanar line are integrated has a high gain of 13 dB at an operation frequency of 21.6 GHz and exhibits wide-band operation and low distortion (see, for example, M. Nishijima et al., 2005 IEEE MTT-S IMS Digest, Session TU4B).
In each of the conventional semiconductor integrated circuit devices including GaN-based FETs described above, however, the coplanar line is used and the ground line is formed on the surface of a chip, so that the problem of limitation on reduction of the chip area arises.
In addition, ground lines provided at both sides of metal interconnection have the same potential, so that other lines such as an air bridge structure needs to be provided. Accordingly, another problem in which the fabrication process becomes complicated arises.
Furthermore, since sapphire is used for a substrate for the growth of a nitride semiconductor layer, heat dissipation is not good and enhancement of electrical characteristics during high-power operation is limited.